An electronic security system is described in U.S. Pat. Nos. 3,180,147 and 3,863,244 for detection of the unauthorized removal of items containing a resonant tag circuit. Such system employs a transmitter providing a repetitively swept range of frequencies driving an antenna which generates a swept electromagnetic field in a zone under surveillance. A resonant tag includes a circuit resonant at a frequency within the swept band and operative in response to the applied field to resonate at its characteristic frequency which is sensed by the receiver portion of the system and processed to provide an output alarm indication of tag presence in the surveillance zone. The receiver includes signal discrimination circuits for distinguishing between an actual tag and spurious signals which could be falsely detected as a tag and therefore cause a false alarm. Preferred signal processing techniques for such electronic security systems are shown in the above-cited patents.
A source of interference is sometimes present in the vicinity of a security system such as described above, which has not been distinguishable by the system from a resonant tag signal. The source of this type of interference is a fixed frequency transmitter producing a signal which beats with the swept frequency of the security system transmitter to cause a beat frequency signal which appears substantially the same as a true tag signal to the signal processing circuits of known systems. As a result, the spurious signal produced by the fixed frequency transmitter is not discriminated against and a false alarm may result.